Image sensor

ABSTRACT

In an image sensor  1  according to an embodiment of the present invention, a plurality of embedded photodiodes PD(m,n) are arrayed. Each of the embedded photodiodes PD(m,n) comprises a first semiconductor region  10  of a first conductivity type; a second semiconductor region  20  formed on the first semiconductor region  10  and having a low concentration of an impurity of a second conductivity type; a third semiconductor region  30  of the first conductivity type formed on the second semiconductor region  20  so as to cover a surface of the second semiconductor region  20;  and a fourth semiconductor region  40  of the second conductivity type for extraction of charge from the second semiconductor region  20;  the fourth semiconductor region  40  comprises a plurality of fourth semiconductor regions  40  arranged as separated, on the second semiconductor region  20.

TECHNICAL FIELD

The present invention relates to an image sensor in which a plurality ofembedded photodiodes are arrayed.

BACKGROUND ART

For example, there is a known image sensor consisting of atwo-dimensional array of light receiving portions with embeddedphotodiodes. In each of the embedded photodiodes, for example, an n-typelow-concentration semiconductor region is formed on a p-type substrateand a thin p-type high-concentration semiconductor region is formed on asurface of this n-type low-concentration semiconductor region. Forreadout of charge, an n-type high-concentration semiconductor region isformed on the n-type low-concentration semiconductor region. Since then-type low-concentration semiconductor region can be completely depletedin this embedded photodiode, charge generated in a pn junction part canbe completely read out and occurrence of leak current is suppressed, soas to achieve excellent S/N ratios in detection of light.

In this embedded photodiode, the sensitivity of detection of light canbe improved by increasing the area of the photosensitive region or thearea of the n-type low-concentration semiconductor region and p-typehigh-concentration semiconductor region. However, the increase in thearea of the n-type low-concentration semiconductor region will lead toincomplete readout of the generated charge, so as to leave a remnantcharge in readout. As a result, image lag will occur.

With regard to this problem, the image sensor described in PatentLiterature 1 has an impurity concentration slope from an edge of then-type low-concentration semiconductor region being the photosensitiveregion in the embedded photodiode toward the n-type low-concentrationsemiconductor region being a transfer electrode for readout of charge,so as to lower a potential gradient. Patent Literature 1 describes thatthis configuration reduces the remnant charge in readout.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-open No.2000-236081

Patent Literature 2: Japanese Patent Application Laid-open No.2006-41189

SUMMARY OF INVENTION Technical Problem

In order to form the potential gradient in the n-type low-concentrationsemiconductor region in the embedded photodiode as in the image sensordescribed in Patent Literature 1, however, it is necessary to form asurface with a stepped impurity concentration distribution, or a p-typehigh-concentration semiconductor region. Since delicate control on ioninjection amounts is required for production of the stepped impurityconcentration distribution, the degree of difficulty in terms of processis high and there is a need for a plurality of photolithography masksand photolithography steps, which might raise production cost.

It is therefore an object of the present invention to provide an imagesensor capable of reducing the remnant charge in readout easier thanbefore.

Solution to Problem

An image sensor of the present invention is an image sensor in which aplurality of embedded photodiodes are arrayed. Each of the embeddedphotodiodes comprises a first semiconductor region of a firstconductivity type; a second semiconductor region formed on the firstsemiconductor region and having a low concentration of an impurity of asecond conductivity type; a third semiconductor region of the firstconductivity type formed on the second semiconductor region so as tocover a surface of the second semiconductor region; and a fourthsemiconductor region of the second conductivity type for extraction ofcharge from the second semiconductor region; the fourth semiconductorregion comprises a plurality of fourth semiconductor regions arranged asseparated, on the second semiconductor region.

Since this image sensor comprises the plurality of fourth semiconductorregions for extraction of charge from the second semiconductor region(photosensitive region) arranged as separated, the distance can beappropriately made short from the fourth semiconductor regions to anedge of the second semiconductor region even if the n-typelow-concentration semiconductor region has a large area. Therefore, asufficient potential gradient to the fourth semiconductor regions can beensured in the embedded photodiode, which can reduce the remnant chargein readout of charge from the second semiconductor region. As a result,it is feasible to suppress the occurrence of image lag.

Since this image sensor is realized by simply forming the plurality offourth semiconductor regions for extraction of charge from the secondsemiconductor region, the remnant charge in readout can be reducedeasier than before.

The aforementioned image sensor preferably comprises a light blockingfilm covering the fourth semiconductor regions and a part of aninterconnection connected to the fourth semiconductor regions, the lightblocking film extending in an array direction.

In the case where the shape of a substantial photosensitive region isnot bilaterally symmetric nor up-and-down symmetric because of thefourth semiconductor regions and the interconnection connected to thefourth semiconductor regions, when incident light impinges over adjacentpixels and on a charge readout line of the embedded photodiode in onepixel, a discrepancy can be made between light detection sensitivitiesof the adjacent pixels because of asymmetry of the substantialphotosensitive region.

However, the aforementioned configuration comprises the light blockingfilm extending in the array direction so as to cover the fourthsemiconductor regions and the interconnection connected to the fourthsemiconductor region, whereby the shape of the photosensitive region canbe made bilaterally symmetric and up-and-down symmetric with respect tothe center axis of the pixel. Therefore, the discrepancy between thelight detection sensitivities of the adjacent pixels can be reduced evenif light impinges over the adjacent pixels.

Advantageous Effects of Invention

The present invention allows the image sensor to reduce the remnantcharge in readout easier than before. As a result, it is feasible tosuppress the occurrence of image lag.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing showing a configuration of an image sensor accordingto an embodiment of the present invention.

FIG. 2 is a drawing showing the first embodiment of pixels shown in FIG.1, which shows a pixel viewed from the front side.

FIG. 3 is a drawing showing cross sections of the pixel along the linesin FIG. 2.

FIG. 4 is a drawing of a pixel in a comparative example of the presentinvention viewed from the front side.

FIG. 5 is a drawing showing a cross section of the pixel along the lineV-V in FIG. 4.

FIG. 6 is a drawing showing the second embodiment of the pixels shown inFIG. 1, which shows a pixel viewed from the front side.

FIG. 7 is a drawing showing cross sections of the pixel along the linesVII-VII in FIG. 6.

FIG. 8 is a drawing showing a case without light blocking films, wherelight impinges over adjacent pixels.

FIG. 9 is a drawing showing a case with light blocking films, wherelight impinges over adjacent pixels.

DESCRIPTION OF EMBODIMENTS

The preferred embodiments of the present invention will be describedbelow in detail with reference to the drawings. In each drawing,identical or equivalent portions will be denoted by the same referencesigns.

FIG. 1 is a drawing showing a configuration of an image sensor accordingto an embodiment of the present invention. The image sensor 1 shown inFIG. 1 is provided with M×N pixels P(m,n) (in M rows and N columns)arranged in a two-dimensional array. Here, M is an integer of not lessthan 2 and m any integer of not less than 1 and not more than M.Furthermore, N is an integer of not less than 2 and n any integer of notless than 1 and not more than N. FIG. 1 is drawn without illustration ofa control section for controlling operations of the respective pixelsP(m,n), a signal processing section for processing signals read out ofthe respective pixels P(m,n), and so on, in order to clearly show thefeature of the present invention. A plurality of embodiments will bedescribed below as examples of the pixels P(m,n) having the feature ofthe present invention.

First Embodiment

FIG. 2 is a drawing showing the first embodiment of the pixels P(m,n)shown in FIG. 1, which shows the pixel P1(m,n) viewed from the frontside, and FIG. 3 is a drawing showing cross sections of the pixelP1(m,n) along the lines in FIG. 2. In FIGS. 2 and 3, the pixel P1(m,n)in the mth row and the nth column is shown on behalf of the M×N pixelsP1(m,n). This pixel P1(m,n) has an embedded photodiode PD1(m,n) and atransistor T1(m,n). FIG. 2 is drawn without illustration ofbelow-described p-type high-concentration semiconductor region 30 in theembedded photodiode PD1(m,n), for easier understanding of the feature ofthe present invention.

The embedded photodiode PD1(m,n) has a p-type substrate 10, an n-typelow-concentration semiconductor region 20 formed on this p-typesubstrate 10, a p-type high-concentration semiconductor region 30 formedon this n-type low-concentration semiconductor region 20, and aplurality of n-type high-concentration semiconductor regions 40 formedon the n-type low-concentration semiconductor region 20. These p-typesubstrate 10, n-type low-concentration semiconductor region 20, p-typehigh-concentration semiconductor region 30, and n-typehigh-concentration semiconductor regions 40 correspond to the firstsemiconductor region, the second semiconductor region, the thirdsemiconductor region, and the fourth semiconductor regions,respectively, described in the scope of claims, and the p-type and then-type correspond to the first conductivity type and the secondconductivity type, respectively, described in the scope of claims.

A p-type impurity concentration of the p-type substrate 10 is, forexample, approximately 10¹⁵ cm⁻³ to 10 ¹⁷ cm⁻³. On the p-type substrate10, the n-type low-concentration semiconductor region 20 is formed so asto be embedded in a portion of the p-type substrate 10.

The n-type low-concentration semiconductor region 20 is of a rectangularshape (e.g., a nearly square shape). For example, the n-typelow-concentration semiconductor region 20 has the thickness of about 0.6μm to 1.0 μm, and an n-type impurity concentration of the n-typelow-concentration semiconductor region 20 is relatively low, about 10¹⁶cm⁻³ to 10¹⁸ cm ⁻³. The p-type high-concentration semiconductor region30 and n-type high-concentration semiconductor regions 40 are formed ona surface of the n-type low-concentration semiconductor region 20.

The p-type high-concentration semiconductor region 30 is formed so as tocover the surface of the n-type low-concentration semiconductor region20 and the thickness thereof is small, 0.2 μm to 0.4 μm. A p-typeimpurity concentration of the p-type high-concentration semiconductorregion 30 is relatively high, about 10¹⁷ cm⁻³ to 10¹⁹ cm ⁻³.

These p-type substrate 10, n-type low-concentration semiconductor region20, and p-type high-concentration semiconductor region 30 form aphotosensitive region and an amount of charge generated according to anintensity of light incident into this photosensitive region isaccumulated in a pn junction part formed by the p-type substrate 10 andthe n-type low-concentration semiconductor region 20 and in a pnjunction part formed by the n-type low-concentration semiconductorregion 20 and the p-type high-concentration semiconductor region 30.

Since the n-type impurity concentration of the n-type low-concentrationsemiconductor region 20 is low as described above, the n-typelow-concentration semiconductor region 20 can be completely depleted,and therefore the charge generated in the pn junction parts can becompletely read out.

By forming the thin p-type high-concentration semiconductor region 30 onthe surface of the n-type low-concentration semiconductor region 20, thep-type high-concentration semiconductor region 30, or the substratesurface can be prevented from being depleted even if the n-typelow-concentration semiconductor region 20 is completely depleted. As aresult, a leak current (dark current), which can be generated due tocharge possibly existing on the substrate surface, can be reduced, whichcan increase S/N ratios in detection of light.

On the other hand, the n-type high-concentration semiconductor regions40 are formed at a plurality of locations (e.g., four locations) so asto be surrounded by the p-type high-concentration semiconductor region30. These n-type high-concentration semiconductor regions 40 are arrayedas separated, near centers of the four sides of the n-typelow-concentration semiconductor region 20. The thickness of the n-typehigh-concentration semiconductor regions 40 is relatively small, 0.2 μmto 0.4 μm, and an n-type impurity concentration of the n-typehigh-concentration semiconductor regions 40 is relatively high, about10¹⁹ cm⁻³ to 10²¹ cm⁻³. These n-type high-concentration semiconductorregions 40 are connected through a contact, a via contact, and aninterconnection 50 to the transistor T1(m,n).

The transistor T1(m,n) is composed of n-type high-concentrationsemiconductor regions DS corresponding to a drain and a source, and agate electrode G. The transistor T1(m,n) is formed adjacently near thecenter of one side of the embedded photodiode PD1(m,n), and, forexample, one of the n-type high-concentration semiconductor regions DSalso serves as one of the n-type high-concentration semiconductorregions 40 in the embedded photodiode PD(n) and is connected to theinterconnection 50 so as to be connected to all the n-typehigh-concentration semiconductor regions 40. The transistor T1(m,n)becomes turned on according to a voltage applied to the gate electrodeG, whereby the charge from the n-type low-concentration semiconductorregion 20 extracted through the n-type high-concentration semiconductorregions 40 can be read from one n-type high-concentration semiconductorregion DS into the other n-type high-concentration semiconductor regionDS.

The interconnection 50 is arranged on the p-type substrate 10 betweenthe n-type low-concentration semiconductor regions 20 in adjacentembedded photodiodes PD1(m,n).

The surface and side faces of the substrate are protected by siliconoxide film 70.

The operational effect of the image sensor 1 of the first embodimentwill be described below in comparison to an image sensor 1X according toa comparative example of the present invention.

The image sensor 1X according to the comparative example of the presentinvention is provided with M×N pixels Px(m,n) arranged in atwo-dimensional array, as the image sensor 1 of the first embodimentshown in FIG. 1 is, and these pixels Px(m,n) are different from those inthe first embodiment in that each pixel has a configuration having anembedded photodiode PDx(m,n) instead of the embedded photodiode PD(m,n).The other configuration of the image sensor 1X is the same as that ofthe image sensor 1. The image sensor 1X is the same as that described inPatent Literature 2.

FIG. 4 is a drawing of the pixel Px(m,n) of the comparative exampleviewed from the front side in a lamination direction and FIG. 5 is adrawing showing a cross section of the pixel Px(m,n) along the line V-Vin FIG. 4. FIG. 4 is also drawn without illustration of the p-typehigh-concentration semiconductor region 30 in the embedded photodiodePDx(m,n).

The embedded photodiode PDx(m,n) of the comparative example is differentin the number of n-type high-concentration semiconductor regions 40 fromthe embedded photodiode PD(m,n) of the first embodiment. Namely, theembedded photodiode PDx(m,n) of the comparative example has only onen-type high-concentration semiconductor region 40 for extraction ofcharge, near the center of the n-type low-concentration semiconductorregion 20.

In the case of this embedded photodiode PDx(m,n) of the comparativeexample, if the n-type low-concentration semiconductor region 20 has alarge area, the distance will be long from the n-type high-concentrationsemiconductor region 40 to the edge of the n-type low-concentrationsemiconductor region 20. For this reason, a potential gradient from theedge of the n-type low-concentration semiconductor region 20 to then-type high-concentration semiconductor region 40 becomes almost nulland it becomes difficult to extract the charge at the edge of the n-typelow-concentration semiconductor region 20, which can leave a remnantcharge in readout. As a result, the image lag can occur.

In the case of the image sensor 1 with the embedded photodiodes PD1(m,n)and pixels P1(m,n) of the first embodiment, however, the plurality ofn-type high-concentration semiconductor regions (fourth semiconductorregions) 40 for extraction of charge from the n-type low-concentrationsemiconductor region (second semiconductor region; photosensitiveregion) 20 are arranged as separated; therefore, even if the n-typelow-concentration semiconductor region 20 has a large area, the distancecan be appropriately made short from the n-type high-concentrationsemiconductor regions 40 to the edge of the n-type low-concentrationsemiconductor region 20. Therefore, a sufficient potential gradient tothe fourth semiconductor regions can be ensured in the embeddedphotodiode PD1(m,n), which can reduce the remnant charge in readout ofcharge from the n-type low-concentration semiconductor region 20. As aresult, it is feasible to suppress the occurrence of image lag.

Since the image sensor 1 of the first embodiment is realized by simplyforming the plurality of n-type high-concentration semiconductor regions40 for extraction of charge from the n-type low-concentrationsemiconductor region 20, the remnant charge in readout can be reducedeasier than before.

In the image sensor 1 of the first embodiment, the n-typehigh-concentration semiconductor regions 40 for extraction of chargefrom the n-type low-concentration semiconductor region 20 are formednear the four sides of the n-type low-concentration semiconductor region20 and the interconnection 50 is arranged on the p-type substrate 10between the n-type low-concentration semiconductor regions 20;therefore, the n-type low-concentration semiconductor region 20 and thep-type high-concentration semiconductor region 30 forming thephotosensitive region are prevented from being covered by theinterconnection 50. As a result, it is feasible to increase an apertureratio of the photosensitive region and to improve the sensitivity ofdetection of light.

Second Embodiment

FIG. 6 is a drawing showing the second embodiment of the pixels P(m,n)shown in FIG. 1, which shows the pixel P2(m,n) viewed from the frontside, and FIG. 7( a) is a drawing showing a cross section of the pixelP2(m,n) along the line VIIa-VIIa in FIG. 6. FIG. 7( b) is a drawingshowing a cross section of the pixel P2(m,n) along the line VIIb-VIIb inFIG. 6. In FIGS. 6 and 7, the pixel P2(m,n) in the mth row and the nthcolumn is shown on behalf of the M×N pixels P2(m,n). This pixel P2(m,n)has an embedded photodiode PD2(m,n) and the aforementioned transistorT1(n). FIG. 6 is drawn without illustration of below-described p-typehigh-concentration semiconductor region 30 in the embedded photodiodePD2(m,n), for easier understanding of the feature of the presentinvention.

The embedded photodiode PD2(m,n) of the second embodiment is differentin the forming positions of the n-type high-concentration semiconductorregions 40 from the embedded photodiode PD1(m,n) of the firstembodiment. Namely, the plurality of n-type high-concentrationsemiconductor regions 40 are arranged as separated at nearly equalintervals up and down and left and right in the n-type low-concentrationsemiconductor region 20. The other configuration of the embeddedphotodiode PD2(m,n) is the same as that of the embedded photodiodePD1(m,n).

The pixel P2(m,n) of the second embodiment is provided with a pluralityof light blocking films 60 (e.g., two films). The light blocking films60 extend in an array direction of the pixels P(m,n) shown in FIG. 1. Inthe present embodiment, the light blocking films 60 extend in the columndirection. The plurality of light blocking films 60 each are arranged soas to cover the n-type high-concentration semiconductor regions 40 andthe interconnection 50 connected to the n-type high-concentrationsemiconductor regions 40 and extending in the column direction. Amaterial of the light blocking films to be used is Al or the like, and apreferred material is one with a light absorbing property, e.g., TiN orthe like which can prevent scattering of light to be detected.

The operational effect by the light blocking films 60 will be describedbelow in detail. FIG. 8 is a drawing showing the case without the lightblocking films 60, where light impinges over adjacent pixels P2(1,1),P2(2,1), and FIG. 9 is a drawing showing the case with the lightblocking films 60, where light impinges over adjacent pixels P2(1,1),P2(2,1),

As shown in FIG. 8, in the case where the pixels P2(m,n) are notprovided with the light blocking films 60 and where incident light Aimpinges over the adjacent pixels P2(1,1), P2(2,1) and on charge readoutlines of the embedded photodiode PD2(1,1) in one pixel P2(1,1), thesensitivity of one pixel P2(1,1) becomes lower by the area of the n-typehigh-concentration semiconductor regions 40 and the interconnection 50extending in the column direction, which can cause a discrepancy betweenthe light detection sensitivities of the adjacent pixels P2(1,1),P2(2,1).

However, since the image sensor 1A with the pixels P2(m,n) of the secondembodiment is provided with the light blocking films 60 extending in thecolumn direction so as to cover the n-type high-concentrationsemiconductor regions 40 and the interconnections 50 extending in thecolumn direction as shown in FIG. 9, the shape of the photosensitiveregion can be made bilaterally symmetric and up-and-down symmetric withrespect to a center axis of the pixel P2(m,n). Namely, it is feasible torelax the asymmetry to be caused by the interconnection 50. Therefore,the discrepancy between the light detection sensitivities of theadjacent pixels P2(1,1), P2(2,1) can be reduced even if the light Aimpinges over the adjacent pixels P2(1,1), P2(2,1).

The present invention can be modified in various ways without having tobe limited to the embodiments described above. For example, the embeddedphotodiodes PD(n) and transistors T(n) are formed directly on the p-typesubstrate 10 in the embodiments, but they may be formed on an n-typesubstrate. In this case, a p-type well is formed on the n-type substrateand the same configuration is formed on this p-type well.

INDUSTRIAL APPLICABILITY

The present invention is applicable to usage to reduce the remnantcharge in readout from the image sensor.

LIST OF REFERENCE SIGNS

1, 1A, 1X image sensor

P(m,n), P1(m,n), P2(m,n), Px(m,n) pixels

PD(m,n), PD1(m,n), PD2(m,n), PDx(m,n) embedded photodiodes

10 p-type substrate (first semiconductor region)

20 n-type low-concentration semiconductor region (second semiconductorregion)

30 p-type high-concentration semiconductor region (third semiconductorregion)

40 n-type high-concentration semiconductor regions (fourth semiconductorregions)

50 interconnection

60 light blocking films

70 silicon oxide film

T(m,n), T1(m,n), Tx(m,n) transistor

DS n-type high-concentration semiconductor regions

G gate electrode

1. An image sensor in which a plurality of embedded photodiodes arearrayed, wherein each of the embedded photodiodes comprises: a firstsemiconductor region of a first conductivity type; a secondsemiconductor region formed on the first semiconductor region and havinga low concentration of an impurity of a second conductivity type; athird semiconductor region of the first conductivity type formed on thesecond semiconductor region so as to cover a surface of the secondsemiconductor region; and a fourth semiconductor region of the secondconductivity type for extraction of charge from the second semiconductorregion, and wherein the fourth semiconductor region comprises aplurality of fourth semiconductor regions arranged as separated, on thesecond semiconductor region.
 2. The image sensor according to claim 1,comprising a light blocking film covering the fourth semiconductorregions and a part of an interconnection connected to the fourthsemiconductor regions, the light blocking film extending in an arraydirection.